Combined light modulation device for tracking users

ABSTRACT

For comfortable viewing of a 3-D scene at various viewing angles, a display having a large tracking range for a variable viewer distance is required. A controllable light-influencing element deflects light in coarse steps in a viewer range. Within said steps, the light is deflected by a further controllable light-influencing element continuously or with fine gradation. The light modulation device is suitable in holographic or autostereoscopic displays for guiding the visibility ranges of the image information to be displayed so as to follow the eyes of the viewers.

The present invention relates to a combined light modulator device for aholographic or an autostereoscopic display with observer tracking. Inthe context of the present invention, a combined light modulator deviceis understood to be a device which changes in a multi-stage process theproperties and/or the direction of light which is emitted by one ormultiple real or virtual light sources.

Here, a virtual light source is a light source which is only seeminglysituated at a certain position, i.e. a light source which appears to bethere as a result of manipulating light of a real light sourceby—typically static—imaging means, such as, for example, a mirror and/orbeam confining means such as apertures.

In the context of this patent application, a holographic display is adisplay device for three-dimensional image data where thethree-dimensional object data of the scene to be represented are encodedin the form of diffraction patterns of the scene to be reconstructed.The reconstruction of a three-dimensional scene in a large visibilityregion at high quality requires both great computing power and ahigh-resolution light modulator.

In document DE 103 53 439 B4, the applicant has thus proposed a methodin which the wave front is only computed for a small visibility regionwhose diameter is only little larger than the diameter of an eye pupilof an observer eye. Consequently, each of the object points to bereconstructed only needs to be encoded in a small region of the lightmodulator in particular sub-holograms.

For this, sufficiently coherent light, which is emitted by at least onelight source, illuminates at least one light modulator and is imaged toat least one observer eye by a field lens. The reconstruction of thethree-dimensional scene for the other observer eye can be generated byalternately switching on at least one other light source in synchronismwith the light modulator while writing a corresponding hologram orcorresponding sub-holograms to the light modulator. Here, colourrepresentation is possible by way of spatial or temporal interleaving(space or time division multiplexing) of the hologram information foreach colour component. To enable the observer to move freely in front ofthe display, the focal regions are tracked to the observer eyes byswitching on further light sources separately. For this, the coordinatesof the eyes of one or multiple observers are continuously determinedwith the help of a position detection system.

Here, the reconstruction of the scene can be adapted to the new observerposition by recalculating of the diffraction pattern. It is alsopossible to provide reconstructions for multiple observers by way oftemporal interleaving (time division multiplexing).

To provide observer tracking along the optical path, the focal plane ofthe focusing unit, and thus the size of the visibility region, ispreferably additionally adapted to the eye positions of the observers.

In an autostereoscopic display (ASD) with observer tracking, it is notdiffraction patterns that are encoded on the light modulator, but ratherare the scene views for the particular eye written directly.

Observer tracking can be realised by way of direct or indirectdisplacement of the light sources. A known example of indirectdisplacement are deflection mirrors.

Numerous other methods of observer tracking are known. Observer trackingcan be achieved, for example, by modifying the optical path in front ofor behind the light modulator which is used for hologram encoding or forstereo representation. In addition to mechanical methods, methods ofchanging reflective, diffractive or refractive properties using adaptiveoptical systems are known.

Further, it is known to use combined tracking methods, i.e. methodswhich take advantage of a light-deflecting function that is static butvaries across the surface area of the light-deflecting means.

In the patent application DE 10 2008 054 438 A1 filed by the applicant,a matrix of electrically controllable fluid cells is proposed forobserver tracking with the fluid cells having additional staticlight-deflecting means which, however, vary across the surface area ofthe matrix in order to realise or at least to support the function of afield lens. These light-deflecting means can, for example, compriserefractive elements, such as prisms or lenses, or diffractive elements,such as volume gratings or blazed gratings, i.e. gratings which areoptimised for a certain wavelength.

The patent application DE 10 2009 028 626 A1 filed by the applicant,whose disclosed content shall be included in full here by way ofreference, teaches to use controllable diffractive gratings for observertracking.

Here, multiple gratings of this kind with the same direction ofdeflection can also be arranged one after another in order to realise alarger deflection angle. Here, it is also possible to arrange at leasttwo controllable deflection gratings one after another which are turnedto one another by a fixed angle in order to achieve a two-dimensionaldeflection. By varying the written grating period, the diffractivegratings can realise a locally different deflection across the surfacearea of the deflection unit in order to realise or at least to supportthe function of a field lens.

In a controllable deflection grating whose grating period is variable soto set a desired diffraction angle, there is a minimum settable perioddue to the spatial resolution with which the deflection grating can becontrolled. If the period is set, for example, using a grid-likeelectrode structure, there are limitations to the width and distance ofthe electrodes caused by the manufacturing process. In addition,electric stray fields or diffusing or diffractive components of thedeflection grating, for example, cause cross-talking among setneighbouring phase values. They can also reduce the diffractionefficiency and thus cause the occurrence disturbing diffused light orlight in higher diffraction orders.

Since in a grid-shaped diffractive structure the diffraction angle isinversely proportional to the periodicity of the diffractive structure,the available angular range and thus the tracking range of a singlediffraction device is limited by the producible electrode pitch.

However, to be able to watch a 3D scene comfortably at various viewingangles, a display is required to have a large tracking range at avariable observer distance. A solution is thus sought which, the limiteddiffraction angle of a deflection element notwithstanding, provides atracking range which is larger than that achievable with such element.

This object is solved according to this invention by the features ofclaim 1.

A light modulator device for a holographic or an autostereoscopicdisplay for the representation of three-dimensional image informationwith at least one real or virtual light source, at least one lightmodulator to which is written encoded image information of the image tobe represented to at least one observer eye of at least one observer, afirst and a second light-affecting means for changing the optical pathof the light which is emitted by the light source, an eye positiondetection system for finding and following at least one eye position ofthe at least one observer of the image information and a systemcontroller for tracking at least one visibility region of the imageinformation based on the eye position information provided by the eyeposition detection system using the first and second light-affectingmeans is characterised by a first light-affecting means, which tracksthe visibility region to the eyes of the observer in large steps withinthe observer range, and by a second light-affecting means, which tracksthe visibility region to the eyes of the observer finely graduated orcontinuously at least within one such large step of the firstlight-affecting means with the help of at least one electricallycontrollable diffraction grating.

Here, the system controller chooses that direction of deflection of thefirst light-affecting means which comes closest to the currentlyselected eye position of the selected observer and sets it in the firstlight-affecting means. The differential angle between this deflectionangle and the actually selected eye position is simultaneously orpromptly computed by the system controller and set in the secondlight-affecting means.

Both eyes of a selected observer can be served by the system controllerby way of time division multiplexing, where the eye position detectionsystem provides the eye position information needed for this.

For a 3D representation, the image content to be represented, i.e. theparticular stereo view or the encoded hologram, is adapted to therespective right or left eye by the system controller for this. The eyeposition detection system can also be designed such that it additionallyserves as a system for detecting the viewing angle so to be able toreconstruct, for example, only those parts of a scene which the observeractually looks at in a system with a large total viewing angle.

Multiple observers can also be served by way of time divisionmultiplexing, which requires fast light modulators and fastlight-affecting means though.

Here, a refresh rate of at least 60 frames per second is required foreach view in the time division multiplex mode to provide anon-flickering representation. If both the three colour components andthe two eyes of each observer are served by way of time divisionmultiplexing, this refresh rate relates to each view of one colourchannel for one eye of one observer.

In particular in a projection system it is possible to use a separatedeflection system for each observer eye, which allows simultaneousrepresentation of image contents for both observer eyes through a beamcombining system.

It is possible by adapting the image content to be represented to theactual observer position through the system controller that the observerseemingly moves around the image contents to be represented when hemoves his head within the visibility region of the display, where thiseffect can also be exaggerated or reduced artificially.

Since it is very complicated to make achromatic diffractive beamdeflecting elements, a colour representation is also realised by way oftime division multiplexing of the individual colour components in apreferred embodiment.

Depending on the actual physical form of the light modulator device, thesecond light-affecting means with the controllable light-deflectinggratings can be disposed in front of or behind the first light-affectingmeans for rough light deflection. A light-affecting means for roughlight deflection, which requires fixed angles of incidence to ensureproper function, such as, for example, volume gratings for diffractivebeam deflection, is preferably arranged in front of the secondlight-affecting means.

One or both of the light-affecting means can be disposed in front of orbehind the light modulator.

They can be designed to be used in conjunction with a transmissive,emissive or reflective light modulator.

A transmissive or reflective light modulator is used in conjunction withan illumination device, where the latter typically emits collimatedlight with which the light modulator is illuminated.

Examples of transmissive light modulators are liquid crystal modulatorson a transparent substrate with a multitude of controllable liquidcrystal cells which are arranged in rows and columns or modulators whichare based on electrowetting cells.

Suitable reflective modulators include, for example, liquid crystalmodulators on a reflective substrate (e.g. LCoS—liquid crystal onsilicon) or micro-mirror arrays (e.g. DMD—digital micro-mirror device)as fast light modulators.

With a transmissive or reflective light modulator, the first and/or thesecond light-affecting means or parts thereof can be integrated into theillumination device.

If the light modulator is, for example, a phase-modulating lightmodulator where complex hologram values are encoded in two (two-phaseencoding) or more phase pixels of the light modulator and where theassociated phase values are thereafter combined by a beam combiner toform an intensity value with defined amplitude and phase value, thenboth light-affecting means are preferably arranged behind the lightmodulator if the beam combiner requires a defined direction of passageof the pencils of light.

Such a beam combiner is proposed, for example, in the hithertounpublished German patent application DE 10 2009 044 910.8.

In an autostereoscopic display with a largely direction-independentamplitude-modulating light modulator or in a holographic display with alargely direction-independent complex-valued light modulator, it can bepreferred, however, to integrate one or both light-affecting meanswholly or partly into the illumination device of the light modulator.

Remaining direction-specific intensity dependencies can preferably beallowed for by the system controller when encoding the imageinformation, so that these dependencies can be compensated.

Emissive light modulators, such as electroluminescence displays orplasma displays do not require an illumination device, because theyactively serve as a light source themselves. Since their individualpixels are mutually incoherent, they are preferably used as lightmodulators in autostereoscopic displays.

In holographic displays, they may be used as a switchable light sourcecombined with a collimation unit of the illumination device of atransmissive display, if the size of a pixel is small enough to exhibita sufficient coherence length.

The two light-affecting means are controlled by the system controllersuch to direct the beams which are emitted by the light sources suchthat the currently represented information for a particular observer eyelies in the viewing range of that eye.

Here, depending on the physical form and type of encoding, the opticalpath may be affected in the horizontal direction only or both in thehorizontal and vertical direction.

Tracking the visibility region in the horizontal direction only greatlycontributes to the simplicity of the arrangement, becauselight-affecting means which can only change the optical path in onedirection suffice.

In a holographic display, the computing power needed for hologramcomputing is substantially reduced when using one-dimensional encodingmethods compared with two-dimensional encoding methods.

In one-dimensional observer tracking, it is possible to use real orvirtual line light sources. They can, for example, be columns of anemissive display combined with an upstream collimation unit in the formof cylindrical lens arrays. In autostereoscopic displays, it is commonto realise horizontal observer tracking only.

By changing the position of a real or virtual light source in front of acollimation unit in the horizontal or vertical direction, the directionof the collimated illuminating pencil of light can be changed in thehorizontal or vertical direction. This can be done, for example, byswitching on or controlling the brightness of individual light points orlight point clusters of a high-resolution matrix of light sources inconjunction with an upstream array of collimation elements, for examplea lens array.

For one-dimensional deflection, it is possible to use illuminatingstripes in conjunction with an array of cylindrical lenses.

A deflection of the light points or light stripes can also be realisedby mechanical or scanning methods.

The size of the light spot in the observer plane can be adjusted byshifting the light sources in the direction of the optical axis of thecorresponding collimation unit. This can also be achieved by acollimation unit whose refractive power is variable and can thus becontrolled accordingly.

A light-affecting means which comprises a light source array withdisplaceable light sources and a corresponding collimation unit can atthe same time serve as a part of the illumination device that is used inconjunction with a transmissive light modulator.

Aberrations in the optical system can be compensated by controlling thebrightness of individual light source points.

If individual light points of the controllable light source array have aclear distance to each other, then the visibility region can be trackedin large steps with them. The mean deflection angle α of a light sourcewhich is situated at a distance l to the object-side principal plane andat a distance a to the optical axis of the collimation unit is here

α=arctan(a/l).

The centre ray of a light source which is situated 10 mm in front of theobject-side principal plane of the collimation unit and which has alateral offset of 2 mm to its optical axis has an inclination of 11.3degrees relative to the optical axis.

As has already been shown above, a light-affecting means can be made upof multiple components. The first and/or the second light-affectingmeans of the light modulator device can be composed of multiplelight-affecting elements with which the beam direction and/or theposition of the real or virtual light sources are changeableindependently of each other.

Multiple electrically controllable deflection gratings with samedirection of deflection can thus wholly or partly be connected in seriesin order to extend the maximum achievable deflection angle or to realisea separate deflection range. To achieve a two-dimensional deflection,one-dimensionally working light-affecting elements can be combined so toform a light-affecting means. This can for example be done in the formof a crossed arrangement.

Here, the individual light-affecting elements of a light-affecting meanscan also be based on different physical principles.

The system controller considers the deflection properties of eachindividual element, so that it is possible to compensate aberrationswhich occur in one element in one or more other elements.

By varying the extent of the position change of the light of the lightsource and/or the extent of the beam direction change in the firstand/or second light-affecting means depending on the point of incidenceof the light on the surface area of the light-affecting means, thefunction of a field lens can be realised to adjust the size of thevisibility region in the observer plane, or the effect of a separatefield lens can be modified.

By way of controlling this position or beam direction change across thesurface of one or multiple light-affecting means through the systemcontroller, the size of the visibility region can be changed variablyand thus, for example, be adapted to a changed observer distance fromthe display, so that the visibility region stays larger than thediameter of the eye pupil but smaller than the eye separation.

For this, the system controller analyses the position and distanceinformation which is provided by the eye position detection system andsets the computed changes in the deflection angles in the correspondinglight-affecting means in addition to the lateral angles which define theposition of the visibility region in the observer plane.

The change in the beam direction in the light-affecting means for roughtracking of the visibility region can comprise both diffractive andrefractive light-affecting elements.

In an embodiment with static light source array, the individual lensesof the collimation unit have a controllable lens effect so that thefocal length and/or the lateral position of the lens vertex can bemodified. Such a controllable lens based on an electrowetting cell hasbeen disclosed, for example, in the European patent EP 1579249 B1.

For beam deflection in large steps, volume gratings to which at leasttwo holograms are written can preferably be used in the firstlight-affecting means. The required volume grating or a master gratingfor further copies can be made by way of writing the holograms with thedesired entrance and exit distributions with the particular workingwavelength. The holograms can also be written in an optical system whichis substantially identical to the application system or which isincluded in the latter (in-situ exposure) in order to compensateaberrations of involved optical components as much as possible.

Volume gratings can be optimised for very narrow angles of incidencewhich only differ slightly from one another and/or for narrow wavelengthranges. Very high diffraction efficiencies of near 100% can be achievedwith this set-up for phase holograms. Here, the volume gratings serve asangle filters, i.e. only the light of a small angular range isdiffracted to the desired direction, and/or as a wavelength filters,where only light of a selected wavelength range is diffracted to thedesired direction. Light of other angles or wavelengths is transmittedthrough the volume grating without being diffracted.

The Bragg condition must be satisfied and the refractive indexmodulation must be chosen accordingly in order to make sure that onlyone diffraction order, i.e. for example the first, the second or ahigher diffraction order, occurs when light passes through the volumegrating. If the refractive index modulation deviates from the optimum,then there will be a non-diffracted portion, i.e. a zeroth diffractionorder, even if the Bragg condition is fulfilled.

Depending on the thickness of the volume grating and the maximumpossible refractive index difference, it may here be necessary toilluminate the grating such that multiple beam interference occurs, i.e.that enough grating layers are passed by the individual light beams.This means that the minimum generated diffraction angle is not toosmall, that it is 30 degrees, for example. This can be achieved byilluminating the volume grating at an angle. A further upstream volumegrating can effect a necessary preliminary deflection should thegeometric arrangement require so.

The thicker the volume grating the greater is its selective effect.

Diffraction processes at volume gratings have been described by HerwigKogelnik in his Theory of coupled waves (H. Kogelnik, “Coupled WaveTheory for Thick Hologram Gratings”, Bell Syst. Techn. J. 48 (1969)2909-2947). A volume grating is considered thick if it has a Q factor

Q=2πdλ/(n ₀Λ²)

that is greater than 10, where d is the thickness of the volume grating,λ the working wavelength of the light in vacuum, Λ the grating constantof the volume grating and n₀ the mean refractive power.

Instead of using a volume grating which is optimised for multiple anglesof incidence and/or multiple wavelengths, multiple volume gratings withsmaller range of functions can be combined in series, i.e. each volumegrating deflects the light into a different direction or focuses it on adifferent point.

In addition to various exit angles, additional field lens functions canpreferably be written to the volume grating during manufacture whichlimit the diameter of the visibility region in the observer plane.

Generally, angle division multiplexing allows different wave fields tobe reconstructed. This corresponds with the principle of holographicreconstruction. This also allows field lenses with different focallengths to be reconstructed. It can also be preferable to reconstructplane waves which propagate into different directions, for example ifthere is a separate field lens.

Here, the foci of the field lenses which are generated by lightdistributions with different angles of incidence do not have to lie inthe same plane as them. For example, light distributions with differentvertical angles of incidence can generate a series of field lenses whosefoci differ, for example, in the horizontal direction, in the horizontaland vertical direction or in the horizontal and vertical direction andin the focal plane.

To generate or support the function of a field lens, it is possible todivide the volume hologram into at least two sub-holograms which lieside by side and each of which satisfying for itself the Bragg conditionwith slightly different exit angle; i.e. the volume grating is dividedinto segments. The manufacture and working principle of such volumeholograms as such is known, for example, from the German patents DE 19700 162 B4 or DE 19 704 740 B4.

For selecting the individual directions which are written to the volumehologram there can be at least one horizontal and/or vertical displacingunit which controllably affects the light emitted by the light sourcessuch that the angle of incidence and/or the point of incidence on thevolume hologram are variable. This unit can here, for example, be partof the backlight unit for a transparent light modulator or part of afrontlight unit for a reflective light modulator. Here, the necessarydisplacement and/or tilt is set by the system controller based on theselected detected eye position. It can be realised in a known mannerusing mechanical, reflective, refractive or diffractive methods. If aflat illumination device is used, the input coupling angle into a planewaveguide can so be varied, for example.

Another preferred embodiment uses for each wavelength range, i.e. forexample for the red, green and blue spectral range, multiple narrow-bandlight sources, which differ only slightly in their principal wavelengthand which are chosen and activated selectively by the system controllerin order to choose or to address the individual diffraction angles inthe at least one volume grating. Such light sources can preferably belasers, for example semiconductor lasers or narrow-band light-emittingdiodes.

Here, the system controller can perform a colour correction of theinformation to be represented, depending on the selected narrow-bandspectral ranges.

Both methods can be combined with each other, so that diffractiongratings for different angles of incidence and for different, closelyneighboured wavelengths can be written to the at least one volumehologram.

In another embodiment, at least one polariser which is switched throughthe system controller is used in conjunction with at least onebirefringent lens in the first light-affecting means for changing thebeam direction in large steps. Such a system is known, for example, fromdocument WO 03 015 424 A2 for 2D/3D switching in an autostereoscopicdisplay.

Here, a birefringent material, for example a liquid crystal mix, isdisposed between two interfaces of two transparent materials which serveas substrate. In this arrangement, at least one interface is curved soto realise the effect of a lens and/or partly inclined in respect to theother interface so to realise the effect of a wedge.

One out of two possible lens and/or deflection effects can be selectedby choosing one out of two possible directions of polarisation by theswitchable polariser. The strength of the lens effect and/or of thewedge angle can vary across the surface area of the deflection and/orfocusing unit. It is further possible to provide the switchablepolariser in a segmented form so to be able to select the two directionsof polarisation locally differently.

The switchable polariser can, for example, be formed by a variableretardation plate with the help of an electrically controllablebirefringent material, which can, for example, comprise a liquid crystalmix as well. Here, the birefringent material is embedded between twosubstrates which are fitted with suitable electrode structures. It ispossible here again to connect multiple of those light-affectingelements in series in order to increase the total effect of thelight-affecting means.

Switchable birefringent light-affecting elements can also be usedinstead of a switchable polariser and a static birefringentlight-affecting element. In such a device, as has been proposed, forexample, for 2D/3D switching in autostereoscopic displays in document WO2007/007 285 A2, the number of required substrates can be reducedcompared with the aforementioned solution. These light-affectingelements can also be segmented and/or arranged in series connection.

Thanks to suitable electrode structures, plane light-affecting elementscan thus also be manufactured where by impressing suitable electricfield distributions a controllable gradient index profile of therefractive index can be generated with which the direction of lightpropagation can be influenced.

In further embodiments, at least one polarisation grating is used toaffect the light. Such a grating only generates the −1^(st), 0^(th) and+1^(st) diffraction order. Here, by using incident circular polarisedlight is its possible to deflect almost 100% of the light into the+1^(st) or −1^(st) diffraction order, depending on the direction ofrotation of the circular polarisation.

Both actively switching polarisation gratings and passive polarisationgratings are known in the art. Polarisation gratings can be manufacturedby way of aligning liquid crystals with adequately prepared surfaces.Such surfaces which serve as structured alignment layers for the liquidcrystals can be generated, for example, by polymerising linearphoto-polymerisable polymers (LPP). For this, the layers are formed, forexample, with interference patterns of circular polarised ultravioletlight, e.g. as emitted by a UV laser.

Without or with only little voltage impressed on an electrode structure,the active polarisation gratings form a periodic grating structure andcan deflect incident circular polarised light at high diffractionefficiency into the +1^(st) or −1^(st) diffraction order, depending onits direction of rotation. If the applied voltage is sufficiently high,the liquid crystals can be aligned such that the grating structure isdestroyed, so that incident light passes through such a light-affectingelement without being deflected, i.e. in the 0^(th) diffraction order.

Passive gratings can, for example, be made by way of polymerising liquidcrystal polymers (LCP). Both active and passive polarisation gratingscan be used in conjunction with a switchable polariser, for example witha switchable retardation plate, in order to select the desireddiffraction order. In an active polarisation grating, the systemcontroller controls both the switchable polariser and the controllablegrating in order to select the desired direction of diffraction. In apassive grating, it is only the switchable polariser that is controlledfor this. It is possible to connect multiple combinations of switchablepolarisers and polarisation gratings as light-affecting means in series.The grating constant of the polarisation grating can be varied acrossthe light entrance surface in order to achieve a locally differentdeflection effect. This variation can be continuous or segmented. Thismakes it possible, for example, to realise or to support the function ofa field lens. It is also possible, for example, to implement cylindricallenses or crossed cylindrical lenses.

In order to enable the system controller to select the actually suitablediffraction angle, the planar switchable polariser can also be of astructured design.

If the grating is used in conjunction with other light-deflectingelements, for example with an upstream field lens and/or otherlight-affecting elements, the grating must be optimised locally for aparticular angle of light incidence. In an active polarisation grating,this can be done by way of local adaptation of the control voltage andthus of the effective birefringence.

In passive polarisation gratings, the hologram which is used duringmanufacture must show a local variation in the grating period.

In a simple polarisation grating, the deflection angle depends on thewavelength. In a colour display, where the individual colour componentsare generated by way of time division multiplexing, this angledifference must be compensated by the system controller using furthercontrollable deflection elements.

Document WO 2008/130 561 A1 also discloses, for example, multiple layersystems of passive polarisation gratings, where the deflection angleremains almost constant across a wide spectral range.

In another embodiment, diffractive gratings whose grating period can bemodified by changing the voltage impressed on a liquid crystal cell areused for rough light deflection. Such a system is described in U.S. Pat.No. 6,188,462 B1, for example. By varying the applied voltage across thesurface of the grating, it is possible here too for the systemcontroller to set a locally different deflection angle in a variableway.

Diffractive phase gratings of the kind provided to realise continuousdeflection in the second light-affecting means can preferably be usedfor rough light deflection as well. In these gratings, the gratingperiods and thus the size of the deflection angle are set by impressinga saw-tooth-shaped voltage profile on a fine electrode structure, i.e.the control voltage rises from electrode to electrode from a base valueto a peak value within a desired or specified grating period. Here, thepeak value determines the maximum phase shift of the light which ismodulated by the liquid crystal layer. Here, the voltage profile doesnot necessarily have to have a clear-cut saw-tooth shape, but shouldrather allow for the characteristic of the voltage-phase relation so toeventually achieve a saw-tooth-shaped phase profile. The smallestpossible grating period, and thus the largest possible deflection angle,is defined by the electrode pitch. Since very fine grids with a pitch offew micrometres to several hundred nanometres are difficult to becontrolled, in particular in large-area deflection gratings, it ispossible to combine electrodes in the deflection grating for roughdeflection and to address these groups of electrodes with a commonsignal if their distance is larger than that which corresponds with thegrating period for the largest possible deflection angle in thedeflection grating for fine deflection. The deflection grating for finedeflection can, for example, have an electrode pitch of severalmicrometres, with each electrode being addressed individually, whereasthe deflection grating for rough deflection has an electrode pitch ofless than one micrometre but where electrodes are combined over a widththat is larger than the electrode pitch of the deflection grating forfine deflection.

Here, combining the electrodes can be segmented across the surface ofthe deflection grating. It is also possible to vary the electrode pitchacross the surface of the grating, e.g. to provide a finer electrodegrid near the edges of the grating so to realise a larger deflectionangle which is required there.

Further light-deflecting elements can preferably be used in atransmissive or reflective mode for beam deflection of pencils of rayswith small cross-section, as they are emitted by small light sources inthe illumination device directly or after beam forming. Acousto-opticmodulators (AOM), for example, allow beam deflection at high speed.Here, the deflection angle can be changed by varying the controlfrequency. The diffraction efficiency can be affected by varying thelevel of the control voltage. AOMs which comprise multiple soundconverters which can be controlled with phase-shifted signals are alsoknown. Thereby, the effective phase grating in the AOM can be inclineddepending on the phase and thus be adapted to changing exit angles sothat the Bragg condition is widely satisfied at the same angle ofincidence in order to realise a high diffraction efficiency at a wideexit angle range and a wide working wavelength range. Such a modulatoris known for example from document U.S. Pat. No. 5,576,880. Since an AOMonly allows small deflection angles to be generated, the angular rangecan be extended by way of providing downstream a volume grating withmultiple exit angles written to it or a respective volume grating stackcomprising individual gratings with different exit angles at angles ofincidence which differ only slightly. Such an arrangement is known forexample from document U.S. Pat. No. 3,980,389.

All optical interfaces of the light-affecting means or light-affectingelements should preferably be fitted out with anti-reflection layers toprevent the occurrence of diffused light. They can have a broad ornarrow spectral and/or angular bandwidth, as is known in the prior art,depending on the actual application.

Suitable apertures for filtering out diffused light and/or diffractionorders which are not used can be disposed in the optical path.Additional means for wave front forming, such as apodisation masks, canbe used as well.

Further possible measures for optimising and specially adapting themethod, including, for example, the use of look-up tables to allow fastcomputation of the control parameters for the deflection angles, the useof joint substrates in a multi-layer design of light-deflectingelements, measures for calibration, error correction, compensation ofthermal effects, compensation of ageing effects or the design of thecontrol circuitry and electrode structures shall be included in thepresent invention and will not be explained in more detail because suchmeasures are apparent to a person who is skilled in the art and knowsthe teaching of the invention described above. All controllablecomponents can also be designed to provide closed-loop control inconjunction with suitable additional sensing devices.

Other known light-deflecting elements than the mentioned electricallycontrollable diffraction gratings can be used as well for continuousobserver tracking.

Now, there are a number of possibilities for embodying and continuingthe teachings of the present invention. To this end, reference is madeon the one hand to the dependent claims that follow claim 1, and on theother hand to the description of the preferred embodiments of thisinvention below including the accompanying drawings. The Figures areschematic drawings, where

FIG. 1 shows a first embodiment of the invention,

FIG. 2 shows a detail of a light-affecting means for observer trackingin large steps with the help of switchable light sources,

FIG. 3 shows a light-affecting means for observer tracking in largesteps with the help of switchable light sources with additional fieldlens function,

FIG. 4 shows a detail of a light-affecting means for observer trackingin large steps by way of light source displacement with the help ofdiffractive deflection gratings in the illumination device,

FIG. 5 shows a detail of an illumination device with a volume gratingand angular multiplexing,

FIG. 6 shows a light modulator device with a reflective light modulator,

FIG. 7 illustrates the generation of two field lenses with the help ofan active liquid crystal grating,

FIG. 8 shows a light modulator device with a transmissive lightmodulator and a liquid crystal phase grating with controllable gratingperiod in conjunction with a multiplex field lens,

FIG. 9 shows a flat backlight device which allows vertical andhorizontal displacement of the wave field which is generated by acollimation unit before it enters a first volume grating,

FIGS. 10 a to 10 c show exemplary effects of two controllable volumegratings as vertical light-affecting elements in FIG. 9, and

FIG. 11 shows a light modulator device with a transmissive lightmodulator and a polarisation grating in conjunction with a switchablepolariser.

FIG. 1 shows schematically a typical embodiment of a light modulatordevice. A light source 100, here a plane light source array, comprises amultitude of individual small light sources 101 to 123 which areswitchable or whose brightness is controllable in an open or closed-loopcontrol process individually or in groups through a system controller900. Here, each single light source 101 to 123 can also comprisemultiple light sources with different principal wavelengths which arealso be independently controllable. The light sources 101 to 123illuminate a plane light modulator 400 through a collimation unit 200,which can comprise an array of individual lenses 201 to 203 or stripesof cylindrical lenses. The lenses 201 to 203 can also be of acontrollable type, so that the focus is variably controllable in one,two or three dimensions by the system controller 900. The device cancomprise an aperture stop 250 which prevents light emitted by one of thelight sources 101 to 123 from passing through multiple lenses 201 to 203of the collimation unit 200. This is of particular importance if thedevice is designed for multi-user operation. In the embodimentillustrated in FIG. 1, a transmissive light modulator 400 is used whichmodifies the amplitude and/or phase of the light in the plane. Thecombination of the controllable light source array 100 and thecollimation unit 200 forms a dynamic illumination device 300.

The light modulator 400 receives its modulation values for the displayof three-dimensional image information from the system controller 900,which computes these values based on input information of the 3D scene902 and on position information of at least one eye position 1100 of atleast one observer of the image information, said position informationbeing provided by an eye position detection system 800. The systemcontroller 900 allows for the characteristics of the light modulator 400and takes into consideration further correction values which result fromthe specific design of the optical system and from the positioninformation. The image information to be displayed, in particular thescene detail to be represented, can also be prepared outside of thesystem controller 900 based on the eye position information 901 which ismade available by the system controller 900 to an external computingunit. The eye position detection system 800, which is known as such inthe art, can comprise, for example, at least one camera and acorresponding signal processing unit, where the signal processing unitcan also be part of the system controller 900. The signal processingunit finds the position of the eye pupils in the particular camera imageand calculates the corresponding spatial coordinates of all observereyes 1100. Other eye position detection systems 800, which work, forexample, with ultrasound, or which use passive or active marks or signalsources which are associated with the observer can be used as well.

Further light-affecting means 501, which are controlled by the systemcontroller 900, can be disposed in the optical path between the lightsources 101 to 123 and the observer eyes 1100. In the illustratedembodiment, the dynamic illumination device 300—alone or in combinationwith the further light-affecting elements 501—forms the light-affectingmeans 500 for rough beam deflection. A second light-affecting means 600is provided in the form of diffractive controllable deflection gratings,where said light-affecting means can also comprise multiplelight-affecting elements, in order to direct the particular visibilityregion 1000 continuously or in fine steps at the particular observer eyeas controlled by the system controller 900 based on the eye positioninformation 901. Referring to the embodiment pictured in FIG. 1, a fieldlens 700 is provided for focusing the visibility region 1000 on theobserver plane, where said field lens can also be designed in the formof a controllable adaptive lens which is controlled by the systemcontroller 900 to adjust the size of the visibility region 1000depending on the distance of the observer eyes 1100 from the lightmodulator 400. The function of the field lens 700 can, however, whollyor partly be integrated into the dynamic illumination device 300 and/orinto further light-affecting elements 501 and/or into light-affectingelements of the second light-affecting means 600.

FIG. 2 shows schematically a detail of an illumination device which isdesigned such to serve as a light-affecting means for rough tracking ofat least one visibility region to the position of at least one observereye using switchable light sources 101 to 103.

A multitude of switchable or controllable light sources 101, 102, 103are situated in front of a collimation unit 200, which can compriserefractive and/or diffractive elements. The desired direction ofdeflection is selected by switching on one of the exemplarily shownlight sources 101, 102 or 103. The deflection angle depends on thedistance of the light source to the optical axis OA of the segment ofthe collimation unit 200 and on its distance to the object-sideprincipal plane of these image segments. In the illustrated embodiment,the light sources 101 to 103 are situated in the object-side focalplane, so that the light leaves the collimation unit 200 parallel. Thislight illuminates the light modulator 400.

FIG. 3 shows schematically an option for observer tracking in largesteps with the help of switchable or controllable light sources 101 to123 with additional field lens function. The individual light sources101 to 123 of a light source 100 which is provided in the form of aplane light source array are arranged asymmetrically behind thecollimation lenses 201 to 203 of a collimation unit 200, so that thelight which is emitted by the light sources 101 to 123 and which passesthrough the light modulator 400 is deflected more strongly towards thecentre of the observer region of the display device near the edges ofthe light modulator 400 than light which is emitted by light sources inthe centre of the collimation unit 200. As shown here, the light sources101 to 123 can be arranged outside the focal plane of the collimationlenses 201 to 203, so that they are imaged to the central observerplane.

The individual light sources 101 to 123 can be composed of individuallyswitchable or controllable sub light sources with different spectraldistributions of the emission characteristics. The individual sub lightsources can be slightly staggered in depth, i.e. located at differentpositions in relation to the optical axis, so to compensate chromaticaberration of the collimation unit in order to allow all colourcomponents to be imaged largely in the same central observer plane.

For this purpose, the refractive power of the collimation lenses 201 to203 can, for example, be variably changeable by the system controller inorder to compensate such chromatic aberration and to adjust the observerplane to the distance of the observer from the display device.

There are a number of further possibilities to realise the function of afield lens. The optical axes of the individual collimation lenses 201 to203 could, for example, be inclined more strongly towards the edge ofthe lens array 200, so that, for example, all optical axes intersect inthe centre of the observer region in the central observer plane. Thelight sources 101 to 123 which are assigned to a certain lens 201 to 203can be disposed at an angle too.

FIG. 4 shows the principle of the observer tracking using diffractivegratings with the example of a detail of an illumination device. Acollimated light source 101 illuminates a switchable or controllablelight-affecting element 501 for beam deflection, which can comprise, forexample, at least one diffractive deflection grating. The latterdeflects the beams of the collimated light source 101 to a differentlocation on a diffusing plate 110, depending on the set deflectionangle. The diffusion profile can be varied locally such that thefollowing collimation lens 201, which can comprise, for example,diffractive and refractive elements, is illuminated as optimally aspossible. The locally varied diffusion profile of the diffusing plate110 can, for example, be generated holographically. The points ofincidence of the pencils of light which are deflected by thelight-affecting element 501 represent deflection-angle-dependentsecondary light sources 111 to 113, which illuminate a region of thelight modulator through the collimation lens 201 of a collimation unit,as shown in FIG. 2 or FIG. 3. The light source 101 can again compriseindividually switchable sub light sources with different spectraldistributions of the emission characteristics. The light-affectingelement 500 can be composed of multiple deflection gratings in order toprovide for a two-dimensional deflection, for example. Suitable aperturestops 250 can prevent light of unused diffraction orders, which canoccur in the light-affecting element 501, or light of the secondarylight sources 111 to 113 which does not fall on the collimation lens 201from illuminating other collimation lenses of the collimation unit andfrom propagating though the illumination device as unwanted stray light.

Controllable diffractive gratings whose grating period can be controlledvariably can preferably be used as deflection gratings in thelight-affecting element 501.

Phase gratings which are based on liquid crystal cells can, for example,be used where variable grating periods and thus deflection angles arewritable with a grid electrode structure.

Moreover, acouto-optic modulators can be used as well.

However, further embodiments are possible, including the use of activeand passive polarisation gratings in conjunction with controllableretardation plates.

Since the distance between the light-affecting means 501 and thediffusing plates 110 can be chosen rather large, it is possible to usein the light-affecting means 501 deflection gratings which can onlygenerate small deflection angles. Only low demands will thus be made onthe required minimum grating period, which simplifies the manufacture ofsuch deflection elements considerably.

The controllable grating can be illuminated at an angle in order toblank out undesired intensities in the zeroth diffraction order. Inorder to realise an optimum deflection range for each workingwavelength, where said deflection ranges are largely overlapping, thegrating can be illuminated at a different, adapted angle for eachworking wavelength range.

Instead of the deflection grating in the light-affecting element 501,other deflection elements can be used as well. Controllableelectro-wetting cells can be used, for example, where the position of ameniscus or the position and shape of a meniscus as an interface of twoliquids with different refractive index can be varied in one or twodirections.

FIG. 5 shows schematically a detail of an illumination device with avolume grating and angular multiplexing of the light sources. Thelight-affecting element 501, which comprises at least one volume grating502 for light affecting, is illuminated from slightly differentdirections by multiple light sources 101 to 103 through collimationlenses 201 to 203. Various reconstruction geometries are staticallywritten to the volume grating 502 of the light-affecting element 501. Ifthe volume grating is illuminated from different directions, differentwave fronts are generated and emitted. The volume grating 502 of thelight-affecting element 501, which can also comprise a stack of multiplevolume gratings 502, can, for example, be illuminated at five angleswith an increment of 0.3°, so that on its exit side five field-lens wavefronts are generated with an angle increment of 12°, for example.

FIG. 6 shows schematically a light modulator device with a reflectivelight modulator 400 for image encoding in conjunction with a frontlightunit. The frontlight unit for illuminating the light modulator 400 withcollimated light comprises a stack of plane light-deflecting elements510, 520. Here, the corresponding deflection function can be selected byactivating a light source 110, 120 which is assigned to the particularlight-deflecting element 510, 520. In this embodiment, the light sources110, 120 are each represented by a laser diode 111, 121 for the redspectral range, a laser diode 112, 122 for the green spectral range anda laser diode 113, 123 for the blue spectral range. The light which isemitted by these light sources 110, 120 passes accordingly assignedcollimation units 210, 220 and is coupled into a plane waveguide 513,523 through at least one volume grating 511, 521 each, where eachcombination of volume grating and plane waveguide is disposed on a jointsubstrate 514, 524. In this embodiment, a hologram each for the red,green and blue spectral range are written to each of the volume gratings511, 521. In optically coherent applications, for example in aholographic display device, the plane waveguide 513, 523 should bechosen to be so thin that light can propagate under one reflection angleonly (mono-mode light waveguide) in order to maintain the coherence ofthe light.

The light is coupled out of the plane waveguide 513, 523 through theaccordingly assigned volume grating 512, 522 and directed in acollimated manner at the reflective light modulator 400. After beingmodulated by the reflective light modulator 400, the light of theselected light source 111 to 113, 121 to 123, is deflected by thecorresponding volume grating 512, 522 into the desired direction or, asshown here in this embodiment, focused on the desired location in theobserver plane. In this embodiment, holograms for each workingwavelength of the light sources 110, 120 are written to the volumegratings 512, 522 too. These holograms are made such that a homogeneousluminous intensity is generated across the entire surface of the lightmodulator 400. For this, the diffraction efficiency must be the higherin the volume grating 512, 522 the farther the output coupling point isaway from the corresponding input coupling grating 511, 521.

At least one additional light-deflecting element 600, which workscontinuously or in fine steps, ensures that, depending on the positionof the observer, light can also be directed at eye positions which donot coincide with the fixed focusing points of the holograms which arewritten to the volume gratings 512, 522. Here, the deflection element600 can support the function of a field lens or fully take on thisfunction. Alternatively, a separate field lens can be disposed, forexample, between the light-deflecting element 600 and the observer.

The collimation units 210, 220, which are assigned to the light sources110, 120, can comprise passive and/or active optical elements 211, 212,221, 222 for beam forming and beam direction changing, where saidelements can affect the light reflectively, diffractively andrefractively. Moreover, they can comprise scanning components, forexample in order to illuminate the input coupling volume gratings 511,521 in stripes.

FIG. 7 shows schematically another embodiment of the invention. Here,one out of two field lenses which are written to a static volume grating533 can be selected by a controllable volume grating 532 as controlledby a system controller (not shown in FIG. 7). A reflectivephase-modulating light modulator 400, which is illuminated withcollimated light by a frontlight unit 300, generates a modulated phasedistribution which carries the image information to be represented. Aspatially amplitude- and phase-modulated wave front 450 is generated bycombining the light which has been modulated by neighbouring pixels ofthe phase-modulating light modulator 400 in a beam combiner 410, saidwave front reconstructing the objects to be represented in thereconstruction space. Here, the object points can be reconstructedreally between the observer and the light modulator 400 and virtuallybehind the light modulator 400. The modulated wave front 450 isdeflected by a defined angle by the static volume grating 531 in orderto generate a suitable or optimal angle of incidence for the followingcontrollable volume grating 532. Here, the exit angles of the individualnarrow-band wavelength ranges of the light sources of the frontlightunit 300 can differ slightly from each other. Depending on how thecontrollable volume grating 532 is controlled, the light passes throughthe latter without being diffracted or is diffracted by its gratingstructure into the first diffraction order. Said controllable volumegrating 532 can be, for example, a polymer dispersed liquid crystalgrating. Here, the desired diffraction pattern is created duringmanufacture by way of local polymerisation when a hologram is inscribed.Depending on the voltage impressed on the electrode structure, therefractive index difference among individual grating elements can becontrolled in such gratings. If the voltage is chosen such that there isno refractive index difference, then the light will pass through thegrating without being diffracted. The refractive index difference in thegrating can be chosen by impressing a suitable voltage on the electrodessuch that almost all light of the currently processed reconstructionwavelength range is diffracted into the first diffraction order.

A static volume grating 533, which can also be provided in the form of amultiplex volume grating, focuses the selected direction on the focalregion 1001 or 1002, respectively. Here, the different angles ofincidence for the individual wavelength ranges can also be allowed for,so that the focal regions of the individual colour components form ajoint focal region.

It is also possible to vary the diffraction angles in the gratings 531and/or 532 locally in order to get a suitable local angle of incidencefor the volume grating 533 so that the required diffraction angle can beset in this grating at high diffraction efficiency. A segmentedarrangement can be used here, too. This arrangement can also be appliedto amplitude-modulating light modulators and complex-valued lightmodulators. Moreover, transmissive modulators can be used as well, thenin conjunction with a backlight unit. In an autostereoscopic display, itis thus possible, for example, to switch between the focal points forthe left and right observer eye. Typically, the arrangement is followedby a light-affecting means for continuous tracking of the foci to theobserver position (not shown).

FIG. 8 shows schematically an embodiment of a light modulator devicewith at least one transmissive phase-modulating light modulator 400 forencoding image information in conjunction with a controllable liquidcrystal phase grating 541. The light modulator 400 is illuminated withsufficiently coherent light by a backlight unit 300. After having beenmodulated by the light modulator 400, the light is formed into aspatially amplitude- and phase-modulated wave front 450 in at least onebeam combiner 410. This wave front hits at least one controllable liquidcrystal phase grating 541 for step-wise deflection of the wave front.For this, the liquid crystal phase grating 541 comprises a multitude ofelectrodes which can be addressed individually or in groups with avariable voltage profile. A Bragg grating is created in the liquidcrystal grating by impressing a saw-tooth-shaped voltage profile withvariable period lengths and variable voltage spikes on the electrodestructure. Due to the saw-tooth-shaped phase profile which is thusgenerated by the grating, this grating acts as a blazed grating for theset direction of deflection if both grating period and phase shift areadapted to the currently processed working wavelength. As a consequence,the light of the wave front is diffracted into the desired direction ofdeflection at high diffraction efficiency.

Generally, the liquid crystal phase grating 541 can generate discrete orcontinuously variable angles for three wavelengths, for example.

In the following field lens, which can include a thin volume grating 542and which comprises a thick volume grating 543, one of the focal regions1001 to 1005 which are written to the thick volume grating 543 isselected by the deflection angle that is chosen by the liquid crystalphase grating 541. Here, the thin volume grating 542, if provided,diffracts the light which comes from the liquid crystal phase grating541 such that for the at least one thick volume grating 543 an optimalor suitable angle of incidence is generated so that the light can bediffracted at high diffraction efficiency in the liquid crystal phasegrating 541.

A light-affecting means 600, which comprises at least one finelystructured diffractive liquid crystal phase grating, serves as alight-affecting means for tracking the selected focal region 1001 to1005 continuously or in fine steps to the position of the selectedobserver eye as controlled by the system controller (not shown in FIG.8). The visibility region from which the reconstruction can be viewed bythe selected observer eye is thus generated.

FIG. 9 shows schematically a light-affecting means for rough beamdeflection, said light-affecting means being integrated into a flatbacklight unit. A light source 100 illuminates a collimation unit 220through a beam widening system 210. Here, the light source 100 can, forexample, comprise an individually controllable laser diode each for thered, green and blue spectral range. The light which is collimated by thecollimation unit 220 is directed at the apertures of an aperture stop120 by corresponding lenses of a lens array 230. The apertures have thefunction of secondary light sources and the aperture stop thus forms alight source array. Further optical components 110 which serve tocondition the light which is emitted by the light source 100 can bedisposed in the optical path. At least one moving diffusing plate forreducing disturbing speckle effects can be disposed here, for example,which modulates the coherent laser radiation with a random phase. Alight source array which comprises, for example, a multitude of laserdiodes of the desired wavelength ranges can also be used instead of thesingle light source 100 and the aperture stop 120. The individualsecondary light sources of the aperture stop 120 are collimated inanother lens array 240 and illuminate a first light-affecting element550 for beam deflection in the vertical direction. Further opticalcomponents 130 which serve to condition the light which is emitted bythe secondary light sources 120 can again be disposed in the opticalpath. For example, at least one static or moving diffusing plate can beprovided to limit the spatial coherence on the exit surface of thebacklight to a suitable degree, so that, for example, multiplesub-holograms to be represented do not influence each other. A secondlight-affecting element 560 can affect the light in the horizontaldirection. It is also possible that the light-affecting elements 550,560 affect the light in another direction than the horizontal orvertical direction, or that they are arranged in a different order.Moreover, the light-affecting elements 550, 560 can be combined in onelight-affecting element with a two-dimensional light-affecting effect.The light which is emitted by the light-affecting means 550 passesthrough a light waveguide 260 and illuminates a first volume grating570. The latter directs the light through another light waveguide 270 ata second volume grating 580. Depending on the angular distribution ofthe light, with its selected wavelength distribution, generated in thecontrollable light-affecting elements 550, 560, the angular rangedesired for illuminating a light modulator (not shown here) is selectedby the volume holograms which are written to the volume gratings 570 and580. Here, the angular distribution by the light-affecting elements 550,560 can be dimensioned such that the entire modulator surface isilluminated at a uniform brightness. The diffraction efficiency of thevolume gratings 570, 580 can vary locally for this, as described above.

The light waveguides 260 and 270 should preferably be made of the samematerial, whose refractive power should differ as little as possiblefrom that of the corresponding volume gratings 570 and 580, in order toavoid reflections at the interfaces. One or both light waveguides 260,270 can also be of a wedge-shaped design. However, they can also be madeof a different material, for example air. In this case, the interfacesmay have to be treated with an anti-reflective coating.

The volume gratings 570 and 580 simultaneously effect an anamorphicenlargement of the illuminating wave field which is generated by thesecondary light sources 120 and collimated by the lens array 240. It isthus possible to use small light-affecting elements 550, 560 for theselection of the hologram functions which are written to the volumegratings 570 and 580. They can be manufactured more easily and at lowercosts than large-area arrangements. Moreover, small-area controllabledeflection gratings can be designed to have a smaller grating constant,so that greater diffraction angles are generatable.

Further optical components, such as optical fibres or tilted mirrors canbe disposed in the optical path between the light source 100 and thelight waveguide 260, for example in order to allow a compact design ofthe entire system.

FIGS. 10 a to 10 c show schematically a possible arrangement and threepossible effects of a light-affecting element from the embodiment thatwas illustrated in FIG. 9 with the example of the light-affectingelement 550 for vertical deflection.

Two controllable liquid crystal gratings 551, 552, which are disposedimmediately one after another, serve as a vertical displacing unit toaffect the incident wave front 150 and to transform it into an exit wavefront 160.

In the example that is illustrated in FIG. 10 a, the two controllableliquid crystal phase gratings 551 and 552 generate both a verticaldisplacement and a change in the angle of the direction of propagationof the wave field 150.

The example which is illustrated in FIG. 10 b demonstrates an expansionand displacement of the centre of the wave field 150.

The example which is illustrated in FIG. 10 c shows a displacement andlocally varying change in the exit angle of the wave field 150.

FIG. 11 shows schematically an embodiment of a light modulator devicewhere the visibility region for the reconstruction of the 3D scene istracked in large steps to the observer eye positions with the help of apassive polarisation grating in conjunction with activepolarisation-modifying light-affecting elements.

A phase-modulating light modulator 400, which is illuminated withsufficiently coherent light by a backlight unit 300, and on which thescene to be reconstructed is encoded, generates together with a beamcombiner 410 a spatially amplitude- and phase-modulated wave front 450.The light of the wave front 450 is given a left-handed or right-handedcircular polarisation by a switchable or controllable polariser 591,which is provided, for example, in the form of a switchable orcontrollable retardation plate, and is directed at the followingpolarisation grating 593. The polarisation grating 593 diffracts thelight—depending on the polarisation direction—to the +1^(st) or −1^(st)diffraction order, respectively, at high diffraction efficiency. Here, avolume hologram 592 is disposed between the switchable polariser 591 andthe polarisation grating 593, said volume hologram 592 diffracting thelight which passes through the switchable or controllablepolarisation-modifying element locally into a direction whichcorresponds with a suitable angle of incidence for the polarisationgrating 593.

A polarisation-modifying element 594, which can also be of a switchableor controllable type, can be disposed behind the polarisation grating593 in order to suppress light which is not deflected into the desireddiffraction order.

A following light-affecting means 600 for deflecting the lightcontinuously or in fine steps directs the light of the modulated wavefront 450 at the eyes of the observer, so that the latter can watch thereconstructed 3D scene.

The arrangement can comprise further passive or activepolarisation-modifying elements which set the required polarisationdirection for following polarisation-dependent elements or to transformlinear polarised light into circular polarised light or vice versa.

The embodiment has a passive polarisation grating 593 whose gratingperiod varies locally continuously or in steps. This makes it possible,for example, to realise the function of a field lens. If thelight-affecting means 600 is in its neutral position, the light of thewave front 450 is directed at one of the two visibility regions 1001 or1002, depending on the status of the switchable or controllablepolarisation-affecting elements 591 or 594. One or both switchable orcontrollable polarisation-affecting elements 591 or 594 can also bestructured locally and switchable or controllable separately in one ortwo directions in order to compensate effects caused by the passageangle and/or of the wavelength range of the currently transmitted light.

Polarisation gratings 593 with uniform grating constant can be used too.They deflect the light of the modulated wave front 450 into one out oftwo directions, which are defined by the +1^(st) and −1^(st) diffractionorder, respectively, depending on the status of thepolarisation-modifying element 591. The function of a field lens canthen be realised by additional passive and/or active optical elements,for example volume gratings.

In stacks which comprise locally controllable polarisation-modifyingelements 591, 594 and passive polarisation gratings 593, the effect thedeflection angle of the elements 591, 593, 594 of stack layers which aredisposed more upstream in the optical path has on the polarisationchange in the respective polarisation-modifying element 591, 594 can becompensated in a wavelength-specific manner through these locallycontrollable polarisation-modifying elements 591, 594.Polarisation-modifying elements 594, 591 which are disposed one afteranother and which belong to different neighbouring stack layers can alsobe combined so to form a joint controllable polarisation-modifyingelement.

Such stacks can be used to generate more than two focal regions ordirections of deflection. The polarisation gratings 593 in the stacklayers preferably have different grating constants at the samehorizontal and vertical position, thus generating different diffractionangles there, in order to realise steps that are as uniform and fine aspossible so to prevent double focal regions.

The number of layers in such a stack can be kept small by usingpolarisation gratings 593 with controllable grating period.

With switchable polarisation gratings 593, the zeroth diffraction ordercan be used as well.

Polarisation gratings 593 which exhibit a diffraction efficiency ofalmost 100% through a wide wavelength range can be manufactured byfinding a suitable combination of the layer thickness, and thus of theoptical retardation, and the twisting angle of the liquid crystalmolecules. However, it is also possible to use stacks of switchablepolarisation gratings 593 with each element being optimised for adifferent wavelength range.

In colour division multiplex mode, it is then possible to only activatethe grating which is optimised for the currently processed spectralrange.

A complex-valued light modulator can be used as an alternative to aphase-modulating light modulator 400 and a beam combiner 410. Further,it is possible to use a reflective light modulator in conjunction with afrontlight unit.

In the mentioned embodiments, it is also possible to use lightmodulators which generate the hologram through a scanning device or touse multiple light modulators. Moreover, a holographic orautostereoscopic display can also comprise multiple separate lightmodulator devices which jointly reconstruct a 3D scene or which jointlygenerate a stereoscopic image.

In all embodiments, all active components can be controlled by a systemcontroller based on observer eye position information which isdetermined by an eye position detection system, while aberrations ofoptical components, thermal effects, local deviations of the wave frontform caused by brightness fluctuations in the illumination device 300and modulation errors in the light modulator 400, for example, can bewidely allowed for and compensated. If necessary, such aberrations canbe quantified in calibration measurements or found actively in real-timemeasurements.

Finally, it must be said that the embodiments described above shallsolely be understood to illustrate the claimed teaching, but that theclaimed teaching is not limited to these embodiments.

1. A light modulator device for a holographic or an autostereoscopic display for the representation of three-dimensional image information with at least one real or virtual light source, at least one light modulator to which is written encoded image information of the image to be represented to at least one observer eye of at least one observer, a first and a second light-affecting means for changing the optical path of the light which is emitted by the light source, an eye position detection system for finding and following at least one eye position of the at least one observer of the image information and a system controller for tracking at least one visibility region of the image information based on the eye position information provided by the eye position detection system using the first and second light-affecting means, wherein the first light-affecting means tracks the visibility region to the eyes of the observer in large steps within the observer range and that the second light-affecting means tracks the visibility region to the eyes of the observer finely graduated or continuously at least within one such large step of the first light-affecting means with the help of at least one electrically controllable diffraction grating.
 2. The light modulator device according to claim 1, wherein the second light affecting means is disposed in front of or behind the first light-affecting means in the direction of light propagation and that the first and/or second light-affecting means are disposed in front of or behind the light modulator.
 3. The light modulator device according to claim 1, wherein the position of the light source and/or the direction of the light beam is changeable with the first and/or the second light-affecting means, where the position of the light source is adjustable in one, two or three dimensions and where the direction of the light beam is adjustable in the horizontal and/or vertical direction.
 4. The light modulator device according to claim 1, wherein the first and/or the second light-affecting means are composed of multiple light-affecting elements with which the beam direction and/or the position of the light source are changeable independent of each other.
 5. The light modulator device according to claim 1, wherein the extent of the position change of the light source and/or the extent of the beam direction change in the first and/or second light-affecting means is variable depending on the point of incidence of the light on the surface area of the light-affecting means, so that—in addition to the tracking function—the function of a field lens with static or variable focal length is realisable or that the function of such a field lens is supported.
 6. The light modulator device according to claim 1, wherein the position change of at least one light source is realisable in the first light-affecting means by mechanically moving the at least one light source and/or by modulating the intensity of multiple light sources at different positions.
 7. The light modulator device according to claim 1, wherein at least one first light-affecting means is provided which affects light diffractively and/or refractively.
 8. The light modulator device according to claim 1, wherein at least one first light-affecting means comprises one or multiple consecutive switchable diffractive gratings with a static or locally variable grating period.
 9. The light modulator device according to claim 1, wherein the first light-affecting means includes a device for changing the positions of the light sources and/or their emission directions and that the first light-affecting means comprises at least one volume hologram to which at least two angleselective diffraction angles and/or field lens functions are written, which are selectable by changing the direction of light incidence.
 10. The light modulator device according to claim 9, wherein at least one horizontal and/or vertical displacing unit and/or a tilting unit are disposed between at least one light source and at least one volume hologram, said units controllably affecting the light which is emitted by the light source such that the angle of incidence and/or the point of incidence on the volume hologram are changeable.
 11. The light modulator device according to claim 1, wherein at least one real or virtual light source is provided which illuminates the first light-affecting means switchably with at least two narrow-band wavelength ranges which are close to each other and that the first light-affecting means further comprises at least one volume hologram to which at least two wavelength-selective diffraction angles and/or field lens functions are written which are associated with those two wavelength ranges and which are selectable by changing the wavelengths of the light sources.
 12. The light modulator device according to claim 9, wherein the first light-affecting means includes at least one volume hologram to which at least two angle-selective and at least two wavelength-selective diffraction angles and/or field lens functions are written.
 13. The light modulator device according to claim 9, wherein at least one of the volume holograms of the first light affecting means comprises at least two volume holograms arranged one after another which are matched to different angles of incidence and/or wavelength ranges in order to deflect light in different directions and/or to focus it on different positions.
 14. The light modulator device according to claim 1, wherein the first light-affecting means comprises at least one switchable retardation plate and at least one birefringent lens in order to change the direction of the light beam.
 15. The light modulator device according to claim 14, wherein at least one birefringent lens is a liquid crystal lens.
 16. The light modulator device according to claim 14, wherein at least one birefringent polarisation grating is provided which has a grating period that is static or that varies across the surface of the polarisation grating in order to change the direction of the light beam.
 17. The light modulator device according to claim 1, wherein the first light-affecting means includes at least one birefringent polarisation grating which has a switchable grating period that is static or that varies across the surface of the polarisation grating in order to change the direction of the light beam.
 18. The light modulator device according to claim 17, wherein at least one birefringent switchable polarisation grating is provided which is optimised for light incidence at an angle and/or which has achromatic properties and which is optimised for at least two wavelengths.
 19. The light modulator device according to claim 1, wherein the second light-affecting means includes at least one variable diffractive grating with which the beam direction changes are settable by way of controllably continuously or locally variably changing a grating period of the diffractive grating.
 20. The light modulator device according to claim 19, wherein at least one of the variable diffractive gratings comprises a liquid crystal layer whose locally static or variable grating period is changed by impressing a voltage profile on an electrode structure. 